Coated articles, electrodeposition baths, and related systems

ABSTRACT

Coated articles, electrodeposition baths, and related systems are described. The article may include a base material and a coating comprising silver formed thereon. In some embodiments, the coating comprises a silver-based alloy, such as a silver-tungsten alloy. The coating can exhibit desirable properties and characteristics such as durability (e.g., wear), hardness, corrosion resistance, and high conductivity, which may be beneficial, for example, in electrical and/or electronic applications. In some cases, the coating may be applied using an electrodeposition process.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/232,261, filed Sep. 14, 2011, which is a continuation-in-part of U.S.application Ser. No. 12/723,020 (now U.S. Pat. No. 9,694,562), filedMar. 12, 2010 and U.S. application Ser. No. 12/723,044, filed Mar. 12,2010, which are incorporated herein by reference in their entirety.

FIELD OF INVENTION

The present invention generally relates to coated articles,electrodeposition baths, and related systems. In some embodiments, thecoatings are metallic and are electrodeposited.

BACKGROUND OF INVENTION

Many types of coatings may be applied on a base material.Electrodeposition is a common technique for depositing such coatings.Electrodeposition generally involves applying a voltage to a basematerial placed in an electrodeposition bath to reduce metal ionicspecies within the bath which deposit on the base material in the formof a metal, or metal alloy, coating. The voltage may be applied betweenan anode and a cathode using a power supply. At least one of the anodeor cathode may serve as the base material to be coated. In someelectrodeposition processes, the voltage may be applied as a complexwaveform such as in pulse deposition, alternating current deposition, orreverse-pulse deposition.

Precious metal and precious metal alloy coatings may be deposited usinga process such as electrodeposition. In some applications, a coating mayat least partially wear off as a result of repeated rubbing against asurface. Such an effect may be undesirable, especially when the coatingis applied at least in part to improve electrical conductivity, sincethis effect can increase the resistance of the coating.

SUMMARY OF INVENTION

Coated articles, electrodeposition baths, and articles are provided.

In one aspect, a bath is provided. The bath comprises silver ionicspecies; tungsten and/or molybdenum ionic species; and sodium hydroxide,wherein the bath is suitable for electrodeposition processes.

In another aspect, a bath is provided. The bath comprises silver ionicspecies; tungsten and/or molybdenum ionic species; and a brightenerselected from the group consisting of 2,2-bipyridine and3-formyl-1-(3-sulphonatopropyl)pyridinium.

In one aspect, an electrodeposition system is provided. Theelectrodeposition system comprises an anode comprising silver; acathode; a bath; and a power supply, wherein the bath comprises tungstenand/or molybdenum ionic species and at least one complexing agent,wherein the bath is associated with the anode and the cathode, whereinthe power supply is connected to at least one of the anode and thecathode, and wherein the surface area of the anode is at least fivetimes the surface area of the cathode.

In one aspect, an article is provided. The article comprises a basematerial; and a coating formed on the base material, the coatingcomprising a silver-based alloy, the silver-based alloy furthercomprising tungsten and/or molybdenum, the silver-based alloy having agrain size of less than about 100 nm, wherein the grain size changes byno more than 30 nm following exposure to a temperature of at least 125°C. for at least 1000 hours.

In another aspect, an article is provided. The article comprises a basematerial; a coating formed on the base material, the coating comprisinga silver-based alloy, the silver-based alloy further comprising tungstenand/or molybdenum, wherein the concentration of tungsten and/ormolybdenum in the silver-based alloy in at least 1.5 atomic percent andthe silver-based alloy has an average grain size of less than 1 micron;and a lubricant layer formed on the coating.

In yet another aspect, an article is provided. The article comprises abase material; a coating formed on the base material, the coatingcomprising a silver-based alloy, the silver-based alloy furthercomprising tungsten and/or molybdenum; and a lubricant layer formed onthe coating, wherein the hardness of the article is greater than about 1GPa and the coefficient of friction is less than about 0.3.

In still yet another aspect, an article is provided. The articlecomprises a base material; and a coating formed on the base material,the coating comprising a silver-based alloy, the silver-based alloyfurther comprising tungsten and/or molybdenum in at least 1.5 atomicpercent, wherein the coating has a porosity of at least 10%.

Other aspects, embodiments, and features of the invention will becomeapparent from the following detailed description when considered inconjunction with the accompanying drawings. The accompanying figures areschematic and are not intended to be drawn to scale. For purposes ofclarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. All patent applications and patentsincorporated herein by reference are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrodeposition system according to an embodiment.

FIG. 2 shows an article according to an embodiment.

FIGS. 3A-3B shows images of articles subjected to a durability test A)without and B) with a lubricant layer, according to some embodiments.

FIGS. 4A-4C show scanning electron micrographs of cross sections ofelectrodeposited silver-alloy coatings comprising A) 2.3 wt %, B) 4.5 wt%, and C) 8.7 wt % tungsten, according to some embodiments.

FIG. 4D shows a plot of the porosity versus wt % of tungsten forelectrodeposited silver-tungsten alloys, according to some embodiments.

FIG. 5A shows a plot of the grain size versus tungsten weight percentfor electrodeposited silver-tungsten alloys, according to someembodiments.

FIG. 5B shows a plot of the contact resistance versus applied load foran electrodeposited silver-tungsten alloy which was heated to 125° C.for 1000 hours, according to an embodiment.

FIG. 6 shows a plot of silver concentration versus time forelectrodeposition baths comprising different anode to cathode surfacearea ratios, according to some embodiments.

FIG. 7 shows a plot of the tungsten content of an electrodepositedsilver-tungsten alloy versus current density, according to someembodiments.

FIG. 8 shows a plot of the pH of electrodeposition baths versus thenumber of days for a precipitate to be observed in the electrodepositionbaths, according to some embodiments.

DETAILED DESCRIPTION

Coated articles, electrodeposition baths, and related systems aredescribed. The article may include a base material and a coatingcomprising silver formed thereon. In some embodiments, the coatingcomprises a silver-based alloy, such as a silver-tungsten alloy. Thecoating may, in some instances, include at least two layers. Forexample, the coating may include a first layer comprising a silver-basedalloy and a second layer comprising a precious metal. The coating canexhibit desirable properties and characteristics such as durability(e.g., wear), hardness, corrosion resistance, and high conductivity,which may be beneficial, for example, in electrical and/or electronicapplications. In some cases, the coating may be applied using anelectrodeposition process.

FIG. 1 shows an electrodeposition system 10 according to an embodiment.System 10 includes a electrodeposition bath 12. As described furtherbelow, the bath includes the metal sources used to form the coating andone or more additives. An anode 14 and cathode 16 are provided in thebath. A power supply 18 is connected to the anode and the cathode.During use, the power supply generates a waveform which creates avoltage difference between the anode and cathode. The voltage differenceleads to reduction of metal ionic species in the bath which deposit inthe form of a coating on the cathode, in this embodiment, which alsofunctions as the substrate.

It should be understood that the illustrated system is not intended tobe limiting and may include a variety of modifications as known to thoseof skill in the art.

The electrodeposition baths comprise a fluid carrier for the metalsource(s) and additive(s). In some embodiments, the fluid carrier iswater (i.e., the bath is an aqueous solution). However, it should beunderstood that other fluid carriers may also be used such as moltensalts, cryogenic solvents, alcohol baths, amongst others. In someembodiments, the fluid carrier is a mixture of water and at least oneorganic solvent (i.e., an aqueous bath may contain at least some organicsolvent). Those of ordinary skill in the art are able to select suitablefluid carriers.

The baths include suitable metal sources for depositing a coating withthe desired composition. When depositing a metal alloy, it should beunderstood that all of the metal constituents in the alloy have sourcesin the bath. The metal sources are generally ionic species that aredissolved in the fluid carrier. As described further below, during theelectrodeposition process, the ionic species are deposited in the formof a metal, or metal alloy, to form the coating. In general, anysuitable ionic species can be used. The ionic species may be providedfrom metal salts. For example, silver nitrate, silver sulfate, silversulfamate may be used to provide the silver ionic species whendepositing a coating comprising silver; sodium tungstate, ammoniumtungstate, tungstic acid, etc. may be used to provide the tungsten ionicspecies when depositing a coating comprising tungsten. In some cases,the ionic species may comprise molybdenum. Sodium molybdate, ammoniummolybdate, molybdenum oxide, etc. may be used to provide the molybdenumionic species when depositing a coating comprising molybdenum. It shouldbe understood that these ionic species are provided as examples and thatmany other sources are possible. Any suitable concentration of a metalspecies may be used, and one of ordinary skill in the art will be ableto select a suitable concentration by routine experimentation. In someembodiments, the ionic species in the bath may have a concentrationbetween 0.1 g/L and 100 g/L, between 5 g/L and 50 g/L, or between 1 and20 g/L.

As described herein, the electrodeposition baths may include one or moreadditives that may improve the electrodeposition process and/or qualityof coatings. For example, the electrodeposition bath may comprise atleast one complexing agent (i.e., a complexing agent or mixture ofcomplexing agents). A complexing agent refers to any species which cancoordinate with the ions contained in the solution. In some embodiments,a complexing agent or mixture of complexing agents may permitcodeposition of at least two elements. For example, a complexing agentor mixture of complexing agents may permit codeposition of silver andtungsten.

The complexing agent may be an organic species, such as a citrate ion, acompound comprising a hydantoin, an imide functional group, or asubstituted pyridine compound. The complexing agent may be an inorganicspecies, such as an ammonium ion. In some cases, the complexing agent isa neutral species. In some cases, the complexing agent is a chargedspecies (e.g., negatively charged ion, positively charged ion). Examplesof complexing agents include citrates, gluconates, tartrates, and otheralkyl hydroxyl carboxylic acids; cyanide; hydantoins (e.g.,5,5-dimethylhydantoin), succinimides (e.g., succinimide), and othercompounds comprising an imide functional group; and substituted pyridinecompounds (e.g., nicotinamide).

Generally, a complexing agent, or mixture of complexing agents, may beincluded in the electrodeposition bath within a concentration range of0.1-200 g/L, and, in some cases, within the range of 40-80 g/L. In oneembodiment, the mixture of complexing agents comprises5,5-dimethylhydantoin, citric acid, and nicotinamide. When thecomplexing agent is a compound comprising an imide functional group, theconcentration of the complexing agent may be within the range 30-70 g/Lor 40-60 g/L. When the complexing agent is an alkyl hydroxyl carboxylicacid, the concentration of the complexing agent may, in some instances,be within the range 1-20 g/L or 5-15 g/L. When the complexing agent is asubstituted pyridine compound, the concentration of the complexing agentmay, in some instances, be within the range 0.5-20 g/L or 0.5-5 g/L.When the complexing agent is a hydantoin, the concentration of thecomplexing agent may, in some instances, be within the range of 50-70g/L. Concentrations outside these ranges may be used, and those ofordinary skill in the art will readily be able to determine suitableconcentrations by routine experimentation.

In some embodiments, ammonium ions may be incorporated into theelectrolyte bath as complexing agents and to adjust solution pH. Forexample, the electrodeposition bath may comprise ammonium ions in therange of 1-50 g/L, and within the range of 10-30 g/L. Otherconcentration ranges may also be suitable.

In some cases, the baths may include at least one wetting agent. Awetting agent refers to any species capable of reducing the surfacetension of the electrodeposition bath and/or increasing the ability ofgas bubbles to detach from surfaces in the bath. For example, thesubstrate may comprise a hydrophilic surface, and the wetting agent mayenhance the compatibility (e.g., wettability) of the bath relative tothe substrate. In some cases, the wetting agent may also reduce thenumber of defects within the metal coating that is produced. The wettingagent may comprise an organic species, an inorganic species, anorganometallic species, or combinations thereof. In some embodiments,the wetting agent may be selected to exhibit compatibility (e.g.,solubility) with the electrodeposition bath and components thereof. Forexample, the wetting agent may be selected to include one or morehydrophilic species, including amines, thiols, alcohols, carboxylicacids and carboxylates, sulfates, phosphates, polyethylene glycols(PEGs), or derivatives of polyethylene glycol, to enhance the watersolubility of the wetting agent. In some embodiments, the wetting agentmay comprise a fluorosurfactant. In some embodiments, the wetting agentmay include Zonyl® FSJ (Dupont), Captsone™ (Dupont), or Triton™ QS-15(Dow).

Any suitable concentration of wetting agent may be used. For example,the concentration of wetting agent may be between 10 microliters/L and2000 microliters/L, between 20 microliters/L and 1000 microliters/L, orbetween 50 microliters/L and 500 microliters/L. Other concentrationranges may also be suitable.

In some embodiments, the baths may include at least one brighteningagent. The brightening agent may be any species that, when included inthe baths described herein, improves the brightness and/or smoothness ofthe electrodeposited coating produced. In some cases, the brighteningagent is a neutral species. In some cases, the brightening agentcomprises a charged species (e.g., a positively charged ion, anegatively charged ion). In one set of embodiments, the brighteningagent may comprise at least one pyridine ring or at least one pyridiniumring. In some embodiments, the brightening agent comprises bipyridine,optionally substituted.

Any suitable concentration of brightening agent may be used. Forexample, the concentration of brightening agent may be between 0.01 g/Land 50 g/L, between 0.01 g/L and 10 g/L, between 0.1 g/L and 5 g/L, orbetween 0.1 g/L and 1 g/L. Other concentration ranges may also besuitable.

In some embodiments, the brightening agent is 2,2-bipyridine or3-formyl-1-(3-sulphonatopropyl)pyridinium. The concentration of2,2-bipyridine in the bath may be between about 0.1 g/L and about 5 g/L,or between 0.1 g/L and about 1 g/L, or between about 0.1 g/L and about0.8 g/L. In a particular embodiment, the brightening agent is2,2,-bipyridine at a concentration between about 0.2 g/L and about 0.6g/L. In a particular embodiment, the brightening agent is3-formyl-1-(3-sulphonatopropyl)pyridinium at a concentration of about 2g/L. In one embodiment, an electrodeposition bath comprises 2,2-pyridineas the brightening agent and Triton™ QS-15 (Dow) as the wetting agent.

Those of ordinary skill in the art would be able to select theappropriate combination of ionic species, wetting agent, complexingagent and/or other additives (e.g., brightening agents) suitable for usein a particular application. Generally, the additives in a bath arecompatible with electrodeposition processes, i.e., a bath may besuitable for electrodeposition processes. One of ordinary skill in theart would be able to recognize a bath that is suitable forelectrodeposition processes Likewise, one of ordinary skill in the artwould be able to recognize additives that, when added to a bath, wouldmake the bath not suitable for electrodeposition processes.

In some aspects, various techniques can be used to monitor the contentsof the electrodeposition baths. For example, the techniques maydetermine the concentration of one or more of the additives in the bathsuch as the brightening agent(s), wetting agent(s), complexing agent(s),etc. If the concentration of the additive(s) is below or above a desiredconcentration, the bath composition may be adjusted so that theconcentration lies within the desired range.

The pH of the electrodeposition bath can be from about 2.0 to 12.0. Insome cases, the electrodeposition bath may have a pH from about 7.0 to9.0, or, in some cases, from about 7.6 to 8.4, or, in some cases, fromabout 7.9 to 8.1. However, it should be understood that the pH may beoutside the above-noted ranges. The pH of the bath may be adjusted usingany suitable agent known to those of ordinary skill in the art. In someembodiments, the pH of the bath is adjusted using a base, such as ahydroxide salt (e.g., potassium hydroxide). In some embodiments, the pHof the bath is adjusted using an acid (e.g., nitric acid).

In some embodiments, the electrodeposition bath comprises a hydroxidesalt. In a particular embodiment, the hydroxide salt is sodiumhydroxide. In some cases, the hydroxide salt is not potassium hydroxide.Without wishing to be bound by theory, the use of sodium hydroxide ascompared to use of potassium hydroxide in an electrodeposition bath maybe advantageous, as it may reduce and/or prevent formation ofprecipitates in the solution. For example, in one embodiment, whenemploying an electrodeposition bath comprising potassium hydroxide, atungsten oxide precipitate was observed, while no precipitate wasobserved under substantially similar conditions using sodium hydroxide.In some cases, when using sodium hydroxide, the electrodeposition bathmay have a pH greater than about 6.5 to 9.0. In some cases, the pH isbetween about 6.5 and about 9.5, between about 6.5 and about 8.5,between about 7.0 and about 8.5, or between about 6.5 and 8.0. In somecases, the pH is less than 9.0, less than 8.5, or less than 8.0.

In one embodiment, an electrodeposition bath comprises between about 8to about 9 g/L silver ionic species, around about 27 g/L tungsten ionicspecies, and has a pH less than about 8, greater than about 6.5, orbetween about 6.5 and 8. In another embodiment, an electrodepositionbath comprises between about 4 to about 5 g/L silver ionic species,around about 60 g/L tungsten ionic species, and has a pH less than about8.5, greater than about 6.5, or between about 6.5 and 8.5.

In some cases, the operating range for the electrodeposition bathsdescribed herein is 5-100° C., 10-70° C., 10-30° C., 25-80° C., or, insome cases, 40-70° C. In some cases, the temperature is less than 80° C.However, it should be understood that other temperature ranges may alsobe suitable.

In general, the electrodeposition baths can be used in connection withany electrodeposition process. Electrodeposition generally involves thedeposition of a coating on a substrate by contacting the substrate withan electrodeposition bath and flowing electrical current between twoelectrodes through the electrodeposition bath, i.e., due to a differencein electrical potential between the two electrodes. For example, methodsdescribed herein may involve providing an anode, a cathode, anelectrodeposition bath associated with (e.g., in contact with) the anodeand cathode, and a power supply connected to the anode and cathode. Insome cases, the power supply may be driven to generate a waveform forproducing a coating, as described more fully below. In some embodiments,at least one electrode may serve as the substrate to be coated.

In some embodiments, an electrodeposition system comprises an anode, acathode, a bath, and a power supply connected to at least one of theanode and the cathode. In some cases, the anode comprises silver (e.g.,wherein the anode provides silver ionic species to the bath) and thebath comprises tungsten and/or molybdenum ionic species and optionally,at least one complexing agent and/or other additives. In suchembodiments, the surface area of the anode to the surface area of thecathode may be selected so as to provide an appropriate amount of silverionic species to the bath. Without wishing to be bound by theory, inembodiments wherein the ratio of the anode surface area to the cathodesurface area is too small, the anode may passivate and the silver ionicspecies in the solution may not be replenished. In some cases, thesurface area of the anode (e.g. comprising silver) is at least about 5times, at least about 6 times, at least about 7 times, at least about 8times, at least about 9 times, or at least about 10 times, the surfacearea of the cathode. In a particular embodiment, the surface area of theanode is at least about 5 times the surface area of the cathode.

An anode comprising silver may be formed essentially of silver (e.g.,greater than 95% silver, greater than 97% silver, greater than 98%silver, greater than 99% silver, greater than 99.5% silver, greater than99.9% silver), or may not be formed essentially of silver. In somecases, an anode comprising silver may comprise silver formed on asubstrate (e.g., a conductive substrate). In some cases, an anodecomprising silver may also comprise at least one additional metal (e.g.,tungsten), wherein each of the additional metals may or may not providemetal ionic species to the bath (e.g., tungsten ionic species).

In general, during an electrodeposition process an electrical potentialmay exist on the substrate to be coated, and changes in applied voltage,current, or current density may result in changes to the electricalpotential on the substrate. In some cases, the electrodeposition processmay include the use of waveforms comprising one or more segments,wherein each segment involves a particular set of electrodepositionconditions (e.g., current density, current duration, electrodepositionbath temperature, etc.). The waveform may have any shape, includingsquare waveforms, non-square waveforms of arbitrary shape, and the like.In some methods, such as when forming coatings having differentportions, the waveform may have different segments used to form thedifferent portions. However, it should be understood that not allmethods use waveforms having different segments.

In some embodiments, a coating, or portion thereof, may beelectrodeposited using direct current (DC) deposition. For example, aconstant, steady electrical current may be passed through theelectrodeposition bath to produce a coating, or portion thereof, on thesubstrate. In some embodiments, the potential that is applied betweenthe electrodes (e.g., potential control or voltage control) and/or thecurrent or current density that is allowed to flow (e.g., current orcurrent density control) may be varied. For example, pulses,oscillations, and/or other variations in voltage, potential, current,and/or current density, may be incorporated during the electrodepositionprocess. In some embodiments, pulses of controlled voltage may bealternated with pulses of controlled current or current density. In someembodiments, the coating may be formed (e.g., electrodeposited) usingpulsed current electrodeposition, reverse pulse currentelectrodeposition, or combinations thereof.

In some cases, a bipolar waveform may be used, comprising at least oneforward pulse and at least one reverse pulse, i.e., a “reverse pulsesequence.” As noted above, the electrodeposition baths described hereinare particularly well suited for depositing coatings using complexwaveforms such as reverse pulse sequences. In some embodiments, the atleast one reverse pulse immediately follows the at least one forwardpulse. In some embodiments, the at least one forward pulse immediatelyfollows the at least one reverse pulse. In some cases, the bipolarwaveform includes multiple forward pulses and reverse pulses. Someembodiments may include a bipolar waveform comprising multiple forwardpulses and reverse pulses, each pulse having a specific current densityand duration. In some cases, the use of a reverse pulse sequence mayallow for modulation of composition and/or grain size of the coatingthat is produced.

A coating may be applied using an electrodeposition process at a currentdensity of at least 0.001 A/cm², at least 0.01 A/cm², or at least 0.02A/cm². Current densities outside these ranges may be used as well. Insome cases, a direct current is employed having a direct current densityof greater than about 10 mA/cm², greater than about 15 mA/cm², greaterthan about 20 mA/cm², greater than about 30 mA/cm², or greater thanabout 50 mA/cm². In some embodiments, a direct current density isgreater than about 15 mA/cm², and at current densities below this level,only silver is deposited. For current which is applied in pulses, thefrequency may be any suitable frequency (e.g., between 0.1 Hertz andabout 100 Hz). Similarly, the voltage may be any suitable voltage (e.g.,between about 0.1 V and about 1 V).

The deposition rate of the coating may be controlled. In some instances,the deposition rate may be at least 0.1 microns/minute, at least 0.3microns/minute, at least 1 micron/minute, or at least 3 microns/minute.Deposition rates outside these ranges may be used as well.

Those of ordinary skill in the art would recognize that theelectrodeposition processes described herein are distinguishable fromelectroless processes which primarily, or entirely, use chemicalreducing agents to deposit the coating, rather than an applied voltage.The electrodeposition baths described herein may be substantially freeof chemical reducing agents that would deposit coatings, for example, inthe absence of an applied voltage.

The electrodeposition systems/methods may utilize certain aspects ofmethods/systems described in U.S. Patent Publication No. 2006/02722949,entitled “Method for Producing Alloy Deposits and Controlling theNanostructure Thereof using Negative Current Pulsing Electro-deposition,and Articles Incorporating Such Deposits,” which is incorporated hereinby reference in its entirety. Aspects of other electrodepositionmethods/systems may also be suitable including those described inU.S.Patent Publication No. 2006/0154084 and U.S. application Ser. No.11/985,569, entitled “Methods for Tailoring the Surface Topography of aNanocrystalline or Amorphous Metal or Alloy and Articles Formed by SuchMethods”, filed Nov. 15, 2007; U.S. Patent Publication No. 20090286103and U.S. patent application Ser. No. 12/120,564, filed May 14, 2008;U.S. application Ser. No. 12/723,020, entitled “Electrodeposition Bathsand Systems”, filed Mar. 12, 2010; and U.S. application Ser. No.12/723,044, entitled “Coated Articles and Methods”, filed Mar. 12, 2010which are incorporated herein by reference in their entireties.

FIG. 2 shows an article 20 according to an embodiment. The article has acoating 22 formed on a base material 24. In some embodiments, thecoating comprises a plurality of layers. In some embodiments, thecoating may comprise a first layer 26 formed on the base material and asecond layer 28 formed on the first layer. Each layer may be appliedusing a suitable process, as described in more detail below. It shouldbe understood that the coating may include more than two layers. Itshould also be understood that the coating may include only one layer.However, in some embodiments, the coating may only include two layers,as shown. In some cases, the coating may be formed on at least a portionof the substrate surface. In other cases, the coating covers the entiresubstrate surface.

In some embodiments, the coating comprises one or more metals. Forexample, the coating may comprise a metal alloy. In some cases, alloysthat comprise silver (i.e., silver-based alloys) are preferred. Suchalloys may also comprise tungsten and/or molybdenum. Silver-tungstenalloys may be preferred in some cases. In some cases, the atomic percentof tungsten and/or molybdenum in the alloy may be between 0.1 atomicpercent and 50 atomic percent; and, in some cases, between 0.1 atomicpercent and 20 atomic percent. In some embodiments, the atomic percentof tungsten and/or molybdenum in the alloy may be at least 0.1 atomicpercent, at least 1 atomic percent, at least 1.5 atomic percent, atleast 5 atomic percent, at least 10 atomic percent, or at least 20atomic percent. Other atomic percentages outside of this range may beused as well.

In some embodiments, the silver-based alloy may form first layer 26 ofthe coating. In some embodiments, second layer 28 comprising one or moreprecious metals may form a second layer of the coating. In some cases,the first layer comprising a silver alloy is formed on the basematerial, and the second layer comprising one or more precious metals isformed on the first layer. Examples of suitable precious metals includeRu, Os, Rh, Re, Jr, Pd, Pt, Ag, Au, or any combination thereof. Gold maybe preferred in some embodiments. In some embodiments, a layer consistsessentially of one precious metal. In some embodiments, it may bepreferable that a layer (e.g., the second layer) is free of tin. Inother cases, a layer may comprise an alloy that includes at least oneprecious metal and at least one other element. The element may beselected from Ni, W, Fe, B, S, Co, Mo, Cu, Cr, Zn, and Sn, amongstothers. For example, a layer may comprise a Ni—Pd alloy, a Au—Co alloy,and/or a Au—Ni alloy.

In some embodiments, the coating may include a layer comprising nickel(e.g., a nickel alloy such as nickel-tungsten). In some cases, the layercomprising nickel may be disposed between the base material and thesilver-based alloy layer. In one embodiment, the coating comprises afirst layer comprising nickel, a second layer comprising thesilver-based alloy, and a third layer comprising one or more preciousmetals, where the first layer is formed on the base material, the secondlayer is formed on the first layer, and the third layer is formed on thesecond layer.

A layer of the coating may have any suitable thickness. In someembodiments, it may be advantageous for a layer to be thin, for example,to save on material costs. For example, a layer thickness may be lessthan 30 microinches (e.g., between about 1 microinch and about 30microinches; in some cases, between about 5 microinches and about 30microinches); in some cases the layer thickness may be less than 20microinches (e.g., between about 1 microinch and about 20 microinches;in some cases, between about 5 microinches and about 20 microinches);and, in some cases, the layer thickness may be less than 10 microinches(e.g., between about 1 microinch and about 10 microinches; in somecases, between about 5 microinches and about 10 microinches). In someembodiments, the thickness of a layer is chosen such that the layer isessentially transparent on the surface. It should be understood thatother layer thicknesses may also be suitable.

The second layer may cover the entire first layer. However, it should beunderstood that in other embodiments, the second layer covers only partof the first layer. In some cases, a second layer covers at least 50% ofthe surface area of a first layer; in other cases, at least 75% of thesurface area of a first layer. In some cases, an element from a firstlayer may be incorporated within a second layer and/or an element from asecond layer may be incorporated into a first layer.

In some embodiments, it may preferable for the first layer to be formeddirectly on the base material. Such embodiments may be preferred overcertain prior art constructions that utilize a layer between the firstlayer and the base material because the absence of such an interveninglayer can save on overall material costs. Though, it should beunderstood that in other embodiments, one or more layers may be formedbetween the first layer and the base material. For example, in someembodiments, a barrier layer may be formed between the base material andthe first layer. The barrier layer, in some embodiments, comprisesnickel. In some cases, the barrier layer comprises nickel-tungsten orsulfamate nickel.

In some embodiments, a lubricant layer may be formed as an upper portionof the coating. The lubricant layer may comprise, for example, anorganic material, a self-assembled monolayer, carbon nanotubes, and thelike. In some cases, the presence of a lubricant layer reduces thecoefficient of friction of the coating as compared to a substantiallysimilar coating but which does not include the lubricant layer. Thelubricant layer may be formed of any suitable material, for examplehalogen-containing organic lubricant, a polyphenyl-containing organiclubricant, or a polyether-containing lubricant. In one embodiment, thelubricant layer is formed of a halogen-containing organic lubricant.Specific non-limiting examples of lubricants include Evabrite™(Enthone), Au lube (AMP), NyeTact® 570H (Nye Lubricants), FS-5 (GabrielPerformance Products), S-30 (Gabriel Performance Products), and MS-383H(Miller-Stephenson). In some cases, the lubricant layer comprises amonolayer formed on the surface of the coating.

Those of ordinary skill in the art will be aware of suitable methods forforming a lubricant layer on a coating. For example, in someembodiments, an article comprising the coating may exposed (e.g., dippedinto) to the lubricant (e.g., optionally in a solution), and the articlemay then be allowed to dry, thereby forming the lubricant layer on theupper portion of the coating.

In some embodiments, an article comprising a lubricant layer formed oncoating (e.g., on a base material) may have a reduced coefficient offriction as compared to a substantially similar article which does notcomprise the lubricant layer. In some cases, the article having thelubricant layer has a co-efficient of friction which is at least twotimes less, at least three times less, at least four times less, atleast five times less, or at least ten times less than an article whichnot having the lubricant layer.

In some cases, an article having a lubricant layer may have better weardurability as compared to a substantially similar article which does nothave a lubricant layer. Those of ordinary skill in the art will be awareof suitable methods to determine the wear durability of a material(e.g., ball-on-plate-type reciprocating friction abrasion test, whereinthe ball and plate both are coated with a layer of the alloy, andoptionally the lubricant layer). For example, in some embodiments,minimal or no wear-through may be observed for an article comprising asilver-based alloy and a lubricant layer over 50 cycles, 100 cycles, 250cycles, 500 cycles, or 1000 cycles, with a 100 g applied load, wherein asubstantially similar article which does not comprise the lubricantlayer may show substantial or complete wear-through.

In some cases, the coating (e.g., the first layer and/or the secondlayer) may have a particular microstructure. For example, at least aportion of the coating may have a nanocrystalline microstructure. Asused herein, a “nanocrystalline” structure refers to a structure inwhich the number-average size of crystalline grains is less than onemicron. The number-average size of the crystalline grains provides equalstatistical weight to each grain and is calculated as the sum of allspherical equivalent grain diameters divided by the total number ofgrains in a representative volume of the body. The number-average sizeof crystalline grains may, in some embodiments, be less than 100 nm. Insome cases, the silver-based alloy has a number-average grain size lessthan 50% of a thickness of the silver-based alloy layer. In someinstances, the number-average grain size may be less than 10% of athickness of the silver-based alloy layer. In some embodiments, at leasta portion of the coating may have an amorphous structure. As known inthe art, an amorphous structure is a non-crystalline structurecharacterized by having no long range symmetry in the atomic positions.Examples of amorphous structures include glass, or glass-likestructures. Some embodiments may provide coatings having ananocrystalline structure throughout essentially the entire coating.Some embodiments may provide coatings having an amorphous structurethroughout essentially the entire coating.

In some embodiments, the coating may be crystalline having aface-centered cubic structure. In some embodiments, the coating may be asolid solution where the metals comprising the coating are essentiallydispersed as individual atoms. Such a structure may be produced using anelectrodeposition process. A solid solution may be distinguished from analternative structure formed, for example, using an electroless processwhere granules comprising a first phase containing a first metal species(i.e., tungsten and/or molybdenum) are dispersed within a coatingcomprising a second phase containing a second metal species (i.e.,silver), the second phase having a different composition and/or crystalstructure than the first phase. In some cases, the solid solution may beessentially free of oxygen.

In some embodiments, the coating may comprise various portions havingdifferent microstructures. For example, the first layer may have adifferent microstructure than the second layer. The coating may include,for example, one or more portions having a nanocrystalline structure andone or more portions having an amorphous structure. In one set ofembodiments, the coating comprises nanocrystalline grains and otherportions which exhibit an amorphous structure. In some cases, thecoating, or a portion thereof (e.g., a portion of the first layer, aportion of the second layer, or a portion of both the first layer andthe second layer), may comprise a portion having crystal grains, amajority of which have a grain size greater than one micron in diameter.In some embodiments, the coating may include other structures or phases,alone or in combination with a nanocrystalline portion or an amorphousportion. Those of ordinary skill in the art would be able to selectother structures or phases suitable for use in the context of theinvention.

Advantageously, the coating (i.e., the first layer, the second layer, orboth the first layer and the second layer) may be substantially free ofelements or compounds having a high toxicity or other disadvantages. Insome instances, it may also be advantageous for the coating to besubstantially free of elements or compounds that are deposited usingspecies that have a high toxicity or other disadvantages. For example,in some cases, the coating is free of chromium (e.g., chromium oxide),which is often deposited using chromium ionic species that are toxic(e.g., Cr⁶⁺). In some cases, the coating may be deposited from anelectrodeposition bath that is substantially free of cyanide. Suchcoating may provide various processing, health, and environmentaladvantages over certain previous coatings.

In some embodiments, the electrodeposited coating (e.g., alloy) may beporous. In some cases, the coating has a porosity of at least 5%, atleast 10%, at least 15%, at least 20%, at least 25%, at least 30%, or atleast 50%. In some cases, the coating has a porosity between about 5%and about 30%, or between about 10% and about 25%. In some cases, for asilver-based alloy comprising tungsten and/or molybdenum, the porositymay vary and/or be controlled based upon the percent tungsten containedin the alloy. In a particular embodiment, for a silver-based alloycomprising tungsten and/or molybdenum in at least 1.5 atomic percent,the coating has a porosity of at least about 10%, or between about 10%and about 25%.

Those of ordinary skill in the art will be aware of methods to determinethe porosity of a coating (e.g., alloy), including, but not limited to,direct measurement of porosity by optical and/or density methods. Insome cases, the porosity may be determined using optical methods,wherein the porosity is determined by obtaining an image of the crosssection of the coating and calculating the area of pores (e.g., whichmay be observed, in some cases, as dark spots). With the assumption thatthe pores are homogenous throughout the coating, a volume fraction ofpores can be calculated.

In some embodiments, metal, non-metal, and/or metalloid materials,salts, etc. (e.g., phosphate or a redox mediator such as potassiumferricyanide, or fragment thereof) may be incorporated into the coating.

The composition of the coatings, or portions or layers thereof, may becharacterized using suitable techniques known in the art, such as Augerelectron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS),etc. For example, AES and/or XPS may be used to characterize thechemical composition of the surface of the coating.

The coating may have any thickness suitable for a particularapplication. For example, the coating thickness may be greater thanabout 1 microinch (e.g., between about 1 microinch and about 100microinches, between about 1 microinch and 50 microinches); in somecases, greater than about 5 microinches (e.g., between about 5microinches and about 100 microinches, between about 5 microinches and50 microinches); greater than about 25 microinches (e.g., between about25 microinches and about 100 microinches, between about 1 microinch and50 microinches). It should be understood that other thicknesses may alsobe suitable. In some embodiments, the thickness of the coating is chosensuch that the coating is essentially transparent on the surface.Thickness may be measured by techniques known to those of ordinary skillin the art.

Base material 30 may be coated to form coated articles, as describedabove. In some cases, the base material may comprise an electricallyconductive material, such as a metal, metal alloy, intermetallicmaterial, or the like. Suitable base materials include steel, copper,aluminum, brass, bronze, nickel, polymers with conductive surfacesand/or surface treatments, transparent conductive oxides, amongstothers. In some embodiments, copper base materials are preferred.

The articles can be used in a variety of applications includingelectrical applications such as electrical connectors (e.g., plug-type).In some embodiments, the coating on an electrical connector includes afirst layer comprising a silver alloy, the first layer disposed on abase material, and a second layer comprising a precious metal, thesecond layer disposed on the first layer. The coating can impartdesirable characteristics to an article, such as durability, hardness,corrosion resistance, thermal stability, and reduced electricalresistivity. These properties can be particularly advantageous forarticles in electrical applications such as electrical connectors, whichmay experience rubbing or abrasive stress upon connection to and/ordisconnection from an electrical circuit that can damage or otherwisereduce the conductivity of a conductive layer on the article.Non-limiting examples of electrical connectors include infraredconnectors, USB connectors, battery chargers, battery contacts,automotive electrical connectors, etc. In some embodiments, the presenceof the first layer of a coating may provide at least some of thedurability and corrosion resistance properties to the coating. In someembodiments, the coating may impart decorative qualities, for example ablue tint and reduced tarnish. Additionally, the presence of the firstlayer may allow the thickness of the second layer to be reduced, therebyreducing the amount of precious metal on the article significantly.

The coatings described herein may impart advantageous properties to anarticle, such as an electrical connector. In some embodiments, thecoating, or layer of the coating, may have a low electrical resistivity.For example, the electrical resistivity may be less than 100microohm-centimeters, less than 50 microohm-centimeters, less than 10microohm-centimeters, or less than 2 microohm-centimeters.

The coating or layer of the coating may have a hardness of at least 1GPa, at least 1.5 GPa, at least 2 GPa, at least 2.5 GPa, or at least 3GPa, or between about 2.0 GPa and about 3.0 GPa. Those of ordinary skillin the art would readily be able to measure these properties. In somecases, a coating comprising a silver-based alloy and a lubricant layermay have a hardness of at least 1 GPa, at least 1.5 GPa, at least 2 GPa,at least 2.5 GPa, or at least 3 GPa and a coefficient of friction ofless than about 1.0, less than about 0.75, less than about 0.5, lessthan about 0.4, less than about 0.3, less than about 0.2, or less thanabout 0.1. In some embodiments, the hardness is between about 2.0 GPaand about 3.0 GPa, and the coefficient of friction is less than about0.3, or between about 0.3 and about 0.1.

The coating or layer of the coating may be thermally stable. In somecases, a coating comprising a silver-based alloy further comprisingtungsten and/or molybdenum and having a grain size of less than about100 nm, exhibits little or no change in grain size upon exposure toelevated temperatures for a substantial period of time. In some cases,the grain size of the coating changes by no more than about 30 nm, nomore than about 20 nm, no more than about 15 nm, no more than about 10nm, or no more than about 5 nm following exposure to a temperature of atleast 125° C. for at least 1000 hours. In some cases, the grain sizechanges by no more than about 30 nm, no more than about 20 nm, no morethan about 15 nm, no more than about 10 nm, or no more than about 5 nmfollowing exposure to a temperature of about 125° C. for at least about1000 hours. The thermal stability may be determined under other suitableconditions, for example, at about 150° C. for at least about 24 hours,at about 200° C. for at least about 24 hours, at about 250° C. for atleast about 24 hours, or at about 200° C. for at least about 120 hours.In addition, the contract resistance of the coating may change by lessthan about 25%, less than about 20%, less than about 15%, less thanabout 10%, or less than about 5%, following exposure to a temperature ofabout 125° C. for at least about 1000 hours. Those of ordinary skill inthe art will be aware of suitable methods to determine the thermalstability of a material. In some cases, the thermal stability may bedetermined by observing microstructural changes (e.g., grain growth,phase transition, etc.) of a material during and/or prior to andfollowing exposure to heat. Thermal stability may be determined usingdifferential scanning calorimetry (DSC) or differential thermal analysis(DTA), wherein a material is heating under controlled conditions. Todetermine changes in grain size and/or phase transitions, in situ x-rayexperiments may be conducting during the heating process.

As noted above, coating 20 may be formed using an electrodepositionprocess. In some cases, each layer of the coating may be applied using aseparate electrodeposition bath. In some cases, individual articles maybe connected such that they can be sequentially exposed to separateelectrodeposition baths, for example in a reel-to-reel process. Forinstance, articles may be connected to a common conductive substrate(e.g., a strip). In some embodiments, each of the electrodepositionbaths may be associated with separate anodes and the interconnectedindividual articles may be commonly connected to a cathode.

In some embodiments, the invention provides coated articles that arecapable of resisting corrosion, and/or protecting an underlyingsubstrate material from corrosion, in one or more potential corrosiveenvironments. Examples of such corrosive environments include, but arenot limited to, aqueous solutions, acid solutions, alkaline or basicsolutions, or combinations thereof. For example, coated articlesdescribed herein may be resistant to corrosion upon exposure to (e.g.,contact with, immersion within, etc.) a corrosive environment, such as acorrosive liquid, vapor, or humid environment.

The corrosion resistance may be assessed using tests such as ASTM B845,entitled “Standard Guide for Mixed Flowing Gas (MFG) Tests forElectrical Contacts” following the Class IIa protocol, may also be usedto assess the corrosion resistance of coated articles. These testsoutline procedures in which coated substrate samples are exposed to acorrosive atmosphere (i.e., a mixture of NO₂, H₂S, Cl₂, and SO₂). Themixture of flowing gas can comprise 200+/−50 ppb of NO₂, 10+/−5 ppb ofH₂S, 10+/−3 ppb of Cl₂, and 100+/−20 ppb SO₂. The temperature andrelative humidity may also be controlled. For example, the temperaturemay be 30+/−1° C., and the relative humidity may be 70+/−2%.

The low-level contact resistance of a sample may be determined beforeand/or after exposure to a corrosive environment for a set period oftime according to one of the tests described above. In some embodiments,the low-level contact resistance may be determined according tospecification EIA 364, test procedure 23. Generally, the contactresistivity of a sample may be measured by contacting the sample under aspecified load and current with a measurement probe having a definedcross-sectional area of contact with the sample. For example, thelow-level contact resistance may be measured under a load of 25 g, 50 g,150 g, 200 g, etc. Generally, the low-level contact resistance decreasesas the load increases.

In some embodiments, a coated article has reduced low-level contactresistance. Reduced low-level contact resistance may be useful forarticles used in electrical applications such as electrical connectors.In some cases, an article may have a low-level contact resistance undera load of 25 g of less than about 100 mOhm; in some cases, less thanabout 10 mOhm; in some cases, less than about 5 mOhm; and, in somecases, less than about 1 mOhm. It should be understood that the articlemay have a low-level contact resistance outside this range as well. Itshould also be understood that the cross-sectional area of contact bythe measurement probe may affect the value of the measured low-levelcontact resistance.

The following example should not be considered to be limiting butillustrative of certain features of the invention.

EXAMPLES Example 1

This example demonstrates coating thickness, tungsten content, grainsize, coating hardness, and contact resistance achieved with varioussamples.

Coatings were electrodeposited on base materials in aqueouselectrodeposition baths using an electrodeposition process. Theelectrodeposition baths contained a silver ionic species, a tungstenionic species, and a complexing agent. The coatings were formed directlyon the base material substrate. Additionally, for samples 28-35, anickel layer was electrodeposited on the substrate prior toelectrodepositing the silver-based alloy.

Tables 1 and 2 show the results obtained for these coatings.

TABLE 1 Thickness, tungsten content, grain size and hardness for varioussamples. Thickness Tungsten Grain Hardness Sample (microns) (atomic %)Size (nm) (GPa)  1 2.9 1.0 22 2.4  2 3.1 1.5 20 N.D.  3 3.4 1.4 15 N.D. 4 3.8 1.7 14 2.4  5 3.7 7.9  5 2.2  6 3.8 7.7  5 2.1  7 4.2 7.1  6 N.D. 8 4.1 6.4  7 1.6  9 N.D. 1.2 N.D. N.D. 10 3.1 1.2 96 2.1 11 4.5 1.6N.D. N.D. 12 4.5 1.3 49 N.D. 13 9.8 1.4 55 2.6 14 7.5 1.4 49 N.D. 15 6.44.9 10 2.9 16 8.2 3.1 25 2.8 17 7.9 4.6 10 1.8 18 2.4 1.8 35 2.4 N.D. =not determined.

TABLE 2 Substrate, tungsten content, and contact resistance for varioussamples. Tungsten Contact Resistance Sample Substrate (atomic %)(milliohms) 19 brass 1.3 4.6 20 brass 1.5 4.2 21 brass 2.9 6.8 22 brass6.9 8.8 23 brass 7.0 7.2 24 brass 1.6 5.9 25 brass 1.9 5.3 26 brass 7.310.4 27 brass 5.5 N.D. 28 Ni/brass 1.8 7.7 29 Ni/brass 2.3 6.6 30Ni/brass 6.8 8.4 31 Ni/brass 5.5 10.4 32 Ni/brass 0.9 6.6 33 Ni/brass1.5 5.9 34 Ni/brass 6.7 8.4 35 Ni/brass 7.4 8.0 N.D. = not determined.

Example 2

This example demonstrates coating wear durability for a material whichcomprises a lubricant layer.

A silver-tungsten alloy was electrodeposited as described above inExample 1, on two round surfaces and two flat surfaces. The alloycomprised about 5 wt % tungsten and the thickness of the coating wasabout 80 microinches. The hardness of the coating was about 2.0-2.5 GPa.Following electrodeposition, a lubricant was formed on one of the roundsurfaces and one of the flat surfaces using simple dip applicationmethods known to those in the art. In this example, the lubricant wasEvabrite™. Wear durability studies were conducting as follows: a coatedround surface was placed in contact with a coated flat surface; the flatand round surfaces were then worn against each other through a linearreciprocating motion As shown in FIG. 3, the article which did notinclude the lubricant layer (FIG. 3A) showed significant wear-throughafter 25 cycles while the article which included the lubricant layer(FIG. 3B) showed essentially no wear-through, even after 100 cycles. Thecoefficient of friction for the article without the lubricant layer wasabout 1.0, whereas the coefficient of friction for the article includingthe lubricant layer was about 0.2.

Example 3

This example demonstrates changes in the porosity of an electrodepositedcoating comprising a silver alloy having varying weight percentages oftungsten.

FIGS. 4A-4C show scanning electron micrographs of cross sections ofsilver-tungsten alloys electrodeposited according to the methodsdescribed in Example 1, comprising A) 2.3 wt %, B) 4.5 wt %, and C) 8.7wt %. FIG. 4D shows a plot of the porosity versus wt % of tungsten forelectrodeposited silver-tungsten alloys, according to some embodiments.

Example 4

This example demonstrates the use of electrodeposition baths containingat least one brightening agent.

Silver-tungsten alloys were electrodeposited according to the methodsdescribed in Example 1. In a first case, the bath contained a2,2-bipyridine brightening agent at a concentration of between about 0.2g/L and 0.5g/L. The 2,2-bipyridine was dissolved in ethylene glycolprior to addition of the brightening agent to the bath. In another case,the bath contained a 3-formyl-1-(3-sulphanatopropyl)pyridiniumbrightening agent was at a concentration of about 2 g/L. In both cases,the coatings were bright at all current densities.

Example 5

This example demonstrates thermal stability of a silver-tungsten alloy.

A silver-tungsten alloy coating was electrodeposited on a base materialas described above in Example 1 comprising a variety of weightpercentages of tungsten. The articles were exposed to elevatedtemperatures for selected periods of time. FIG. 5A shows a plot of thegrain size (nm) versus tungsten weight percent for the alloys. FIG. 5Bshows a plot of the contact resistance versus applied load for the asilver-tungsten coating with Evabrite™ lubricant applied which washeated to 125° C. for 1000 hours.

Example 6

This example demonstrates variation of the ratio of cathode to anodesurface areas.

Silver-tungsten alloy coatings were electrodeposited on a base materialas described above in Example 1, wherein the silver ionic species wereprovided to the bath from a consumable silver anode. In this example,the surface area of the anode was 3.5 or 5 times the surface area of thecathode. At a surface area ratio of anode:cathode of 3.5:1, the anodepassivated and the silver ionic species in the solution were notreplenished. At a surface area ratio of anode:cathode of 5:1, the silverconcentration remained approximately constant (see FIG. 6).

Example 7

This example demonstrates variation of the pH of an electrodepositionbath and its relation to tungsten content in the alloy.

A silver-tungsten alloy coating was electrodeposited on a base materialas described above in Example 1. The pH of the electrodeposition bathwas adjusted using sodium hydroxide. A plot of the tungsten contentversus current density is shown in FIG. 7.

Example 8

This example demonstrates variation of the additive to adjust the pH ofthe electrodeposition bath.

Silver-tungsten alloy coatings were electrodeposited from various bathson base materials as described above in Example 1. The pH of theelectrodeposition baths were adjusted using sodium hydroxide, sodiumcarbonate, potassium hydroxide, or potassium carbonate. No precipitation(e.g., of tungsten oxide) was observed for the bath containing sodiumhydroxide or sodium carbonate. In contrast, precipitation was observedin the baths containing potassium hydroxide or potassium carbonate (seeFIG. 8).

What is claimed:
 1. A bath, comprising: silver ionic species; tungstenand/or molybdenum ionic species; and sodium hydroxide, wherein the bathis suitable for electrodeposition processes.